There isn't really any kind of car wash soap that is going to repell dirt. Wax can help keep your paint cleaner by having a slicker surface and letting contaminants blow off. The main purpose of car wash soap is not to protect the paint, but to safely clean the paint. You want a car wash that has lots of lubricants. Washing & Drying your vehicle is usually when the most damage is done to your paint, which is why I never go to automatic car washes. I do it myself. Big Soapy suds are nice, but that doesn't necessarily mean anything.
If I had to stay with off the shelf stuff I'd probably get Meguiars NXT or Gold Class. But I'd rather order some real good wash soap like
Adam's,
P21S, ONR, or PoorBoys